This invention relates to optical transport networks and, more particularly, to optical channel overhead.
It has been determined that it is desirable to provide some optical channel associated overhead. That is, overhead that must follow a particular optical channel connection. Some requirements of such optical channel overhead include the ability to obtain correct validation of connectivity integrity, enhanced assessment of transmission quality, i.e., performance monitoring, and detection and indication of transmission defects.
It is also very desirable that the optical channel overhead be transmitted in such a manner that it is independent of the optical channel client signal type.
In providing these desirable features, it is important that misrouting be avoided of the client signal payload and overhead, scalability issues be avoided related to the adding of optical channels as each optical channel provides its own overhead and advantage is taken of low latency communications between optical channel termination points.
It has been proposed to employ so-called sub-carrier modulation, which employs pilot tones or some other modulation technique to carry the optical channel overhead. However, such modulation techniques add undesirable noise to the optical channel and, hence, the client signal. Consequently, there is a trade off between the level of noise added and the client signal data rate that can be realized. Indeed, the pilot tones compete for spectrum with the low frequency portion of the spectrum of the optical channel client signal. This leads to use of pilot tone frequencies that are dependent on the optical channel client signal.
Additional possible problems resulting from using such sub-carrier or other modulation techniques include: the channel associated optical channel overhead being bit rate constrained in order to minimize impairments to the client signal; the sub-carrier modulation technique frequency and bit rate being dependent on the client signal type; the sub-carrier modulation technique actually ensuring an undesirable level of performance degradation of the client signal; and the use of the sub-carrier modulation technique limiting sub-layer monitoring.
These and other problems and limitations of prior proposed techniques for carrying optical channel overhead are overcome by employing a so-called digital xe2x80x9cwrapperxe2x80x9d in conjunction with an optical channel client signal payload envelope to carry optical channel associated optical channel overhead. That is, the digital wrapper transports optical channel associated optical channel overhead and other monitoring information. This is realized by adding additional capacity, i.e., bandwidth, to the client signal payload envelope. The additional capacity is added xe2x80x9caroundxe2x80x9d the payload envelope and the client signal floats in the payload envelope. This is effected in such a manner that the digital wrapper is independent of the type of client signal that is being transported on the optical channel. Indeed, as such the optical payload envelope is essentially a client optical signal independent, constant bit-rate channel.
Moreover, in addition to the digital wrapper providing capacity for the optical channel overhead, it can also be employed to provide a forward error correction capability.
Additionally, the added digital wrapper readily provides access for performance monitoring and, especially, access to electrical BER (bit error rate).
Technical advantages of employing a digital wrapper, in accordance with the invention, are that it is independent of the client signal type and does not degrade the client signal.